Opiate receptors are also receptors for naturally occurring peptides called endorphins or enkephalins. Different opiate receptor subtypes like those in the pancreas often mediate opposite physiological effects. Differential ionic and GTP sensitivity of receptor subtypes in vitro reflect the extent of receptor coupling with ionophores and other effectors. In vivo assay of neurotransmitter receptors, capable of detecting altered sensitivity states, has been accomplished in male rats and in the future will be applicable to clinical studies in man.